1. Field of the Invention
The present invention relates to a surface acoustic wave device which has been improved both in propagation velocity (V) of surface acoustic wave to be used therein, and in electro-mechanical coupling coefficient (K.sup.2) thereof.
2. Related Background Art
A surface acoustic wave device (hereinafter, mainly referred to as "SAW device"), i.e., a device which utilizes surface acoustic wave (hereinafter, mainly referred to as "SAW") propagating along the surface of a solid material, has various features or advantages common to electro-mechanical functional parts or devices. Such features are as follows:
(1) It has a small size and is light in weight. PA1 (2) It has excellent resistance to vibration and to impact to be applied thereto. PA1 (3) It is liable to provide little dispersion or scattering in product quality even in the mass production thereof, and therefore it is liable to provide a high reliability. PA1 (4) When a circuit including a SAW device is constituted, the SAW device may be mounted or assembled to such a circuit in a so-called "adjustment-free" manner (or in a manner such that substantially no adjustment of the SAW device is required), and therefore the actual mounting of the SAW device may easily be automated or simplified.
In addition to the above-mentioned features common to the electro-mechanical functional parts or devices, the SAW device further has various advantages such that it has excellent stability to temperature, has a long life (or service life), and has an excellent phase characteristic. Accordingly, the SAW device may suitably be used for a wide variety of purposes such as frequency filter, resonator, delay devices signal processing device, convolver, and function element for optoelectronic devices.
In the field of communication technology inclusive of satellite communication system, mobile communication system, etc., along with recent multi-channel and high-frequency development, there has strongly been desired the development of a SAW device which is operable in a higher frequency region (e.g., GHz band) in the field of the above-mentioned SAW device.
In general, the operating frequency (f) of a SAW device may be determined on the basis of a relationship of f=V/.lambda., wherein V denotes the propagation velocity of SAW to be used in the SAW device, and .lambda. denotes the wavelength of the SAW. The wavelength .lambda. depends on the pitch (or period) of an interdigital transducer or electrode having a comb-like shape (hereinafter, referred to as "IDT") constituting the SAW device, as described hereinafter. However, in view of a certain limit in the field of microfabrication technique such as photolithography, it is difficult to extremely shorten the wavelength .lambda. of the SAW to be utilized in the SAW device. Accordingly, for the purpose of high-frequency development in the SAW device, it is desirable to increase the propagation velocity V of the SAW.
In addition, in the field of communication technology such as satellite communication system and mobile communication system as described above, mainly from a viewpoint of actual mounting of a SAW device, it has been desired to improve the entirely of the device so as to further reduce the energy consumption therein and to attain further miniaturization thereof. As a result, in addition to the high-frequency development as described above, it has been desired to improve the electro-mechanical coupling coefficient (K.sup.2) of the SAW device, i.e., an index to conversion efficiency at the time of the conversion of electric energy into mechanical energy in the device.
Accordingly, particularly in a SAW device to be desirably used widely in recent years, it has strongly been demanded that not only the propagation velocity V of the SAW to be utilized in the SAW device should be increased (for example, V.gtoreq.about 7000 m/s), but also the mechanical coupling coefficient (K.sup.2) of the SAW device should be increased (for example, K.sup.2 .gtoreq.2%).
There is known a SAW device having a laminate structure comprising a diamond layer and an LiNbO.sub.3 layer disposed thereon (Japanese Laid-open Patent Application (KOKAI) No. 62911/1989 (i.e., Sho 64-62911). The above-mentioned LiNbO.sub.3 has an advantage such that the chemical stability (i.e., resistance to acid, resistance to alkali, etc.) thereof is superior to that of another piezoelectric material such as ZnO. However, in view of the crystallinity or crystal structure of the LiNbO.sub.3 material itself, it is not easy to form a thin film of LiNbO.sub.3 which is capable of providing a desirable piezoelectric property as a component constituting a SAW device. Particularly, in the prior art, it has not been known what kind of LiNbO.sub.3 thin film may provide a desirable characteristic as a SAW device in relation with a certain electrode arrangement or electrode structure (electrode width, etc.).